LU-13594 obdclass: Add OOM handler for obdclass

LU-13594 obdclass: Add OOM handler for obdclass

This patch adds OOM handler for obdclass. The handler
currently only prints max memory that was used by obdclass
along with current memory being used before attempting
to kill the user process.

Currently, when the handler is kicked in the output under
dmesg would look like:

Output:
~~~~~~~~
...
Mar 21 07:02:02 devbox kernel: obd_memory max: 244859953, obd_memory current: 0
...

Test-Parameters: trivial
Signed-off-by: Arshad Hussain <arshad.hussain@aeoncomputing.com>
Change-Id: I0259f800b1f219ff3427f1d2a17b6a874dd456d3
Reviewed-on: https://review.whamcloud.com/42121
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Aurelien Degremont <degremoa@amazon.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>